Formation of highly luminescent nearly monodisperse carbon quantum dots via emulsion-templated carbonization of carbohydrates

Abstract

Highly luminescent nearly monodisperse carbon quantum dots (CQDs) are synthesized by facile emulsion-templated carbonization of low cost and non toxic carbohydrates excluding the size selection procedure. The present method is further combined with in situ nitric acid treatment to offer high quantum yields up to 53% which, to our best knowledge, is unprecedented in the past.

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Carbon quantum dots (CQDs), as one branch of carbon nanocrystals, are emerging as a new type of fluorescent material which offers strong potential for cost reduction and biocompatibility vis-à-vis inorganic nanocrystals.¹ The structure of CQDs is basically a quasi-spherical graphitic nanoparticle in which the diameter is less than 10 nm. Among a variety of synthetic routes to CQDs reported thus far, bottom-up methods facilely offer highly crystalline carbogenic nanoparticles generally derived from the pyrolysis of carbohydrates in aqueous solutions.^2-15 These methods have distinctive virtues such as low cost (abundant) molecular precursors, low temperature, and sustainability; however, most of them require strong acids, hydrothermal conditions (high pressure), or special equipment. In contrast to inorganic nanocrystals, formation of CQDs kinetically deviates from the La Mer model due to the irreversibility of carbonization.¹⁶ Since the dissolution of a nanocrystal to monomers (the reverse reaction) plays a critical role in focusing of the size distribution, CQDs have a rather broad size distribution compared to inorganic nanocrystals. Furthermore, their slower nucleation rate than the growth rate at common carbonization temperatures spurs the formation of bulk crystals or chunks which deteriorate the photoluminescence quality and the product yield. In this regard, exhaustive size selection procedures (e.g. liquid chromatography) are usually exploited to obtain the desired size, which imposes limitations on the practical application of CQDs. Thus, a different approach should be taken to form uniformly sized CQDs irrespective of the La Mer model.

In this work, we have demonstrated for the first time the facile synthesis of nearly monodisperse CQDs via emulsion-templated carbonization of carbohydrates. Here, aqueous solutions of carbohydrates are emulsified in an aliphatic alcohol in which the hydroxyl functional group could stabilize the emulsions against coalescence. Since each emulsion contains a limited amount of carbohydrate molecules, the formation of undesirable bulk structures would be prevented. Also, we have incorporated inorganic acids into the formation of the CQDs in an attempt to enhance the quantum yield of the visible light emission.

The nearly monodisperse CQDs were synthesized via carbonization of glucose molecules allocated in water-in-oil emulsions (Scheme 1). To form the emulsions, a 10 wt% glucose solution (aq.) was mixed with 1-octanol and the mixture was aged at 80 °C under vigorous stirring for 1 h. To the mixture hexadecylamine was then added and the temperature was elevated to 160 °C under argon to initiate the carbonization. The as-prepared CQDs were purified and finally dissolved in octane. The detailed procedure is provided in the ESI.†

Scheme 1 Schematic illustration of the formation of nearly monodisperse CQDs via emulsion-templated carbonization of glucose.

Formation mechanisms of carbogenic materials from carbohydrates have been extensively investigated previously; nonetheless, the reaction path still remains undefined due to the presence of diverse dehydrated intermediates and the limit of techniques identifying all the glucose-derivable compounds.^17–22 One plausible mechanism could be that glucose is initially dehydrated to furfural intermediates such as 5-hydroxymethyl-furfural-1-aldehyde and these furan compounds are subsequently condensed to form carbon products.^19–22 The time evolution of the vibrational infrared absorption of furan (ether groups, 1300 cm⁻¹) indicates that the formation of the CQDs may conform this mechanism (Fig. 1). The formation of the graphitic structure is designated by the vibrational stretching of C=C (1500 and 3000 cm⁻¹). Also, hexadecylamine grafts the surface of the CQDs to form amide groups which can be confirmed by their broad vibration peak (1600 cm⁻¹). The reaction temperature and time significantly affect the formation of the CQDs. Once the solution was treated below 140 °C, no CQDs were formed though the color of the solution turned to yellow. The formation of the CQDs was completed rather more rapidly above 160 °C (within 30 min) than other hydrothermal reactions. This can be attributed to the evaporation of water in this method which spurs the condensation.

Fig. 1 Infrared spectra of (a) glucose, (b) an aliquot taken during the formation, (c) the hexadecylamine-capped CQDs. The aliquot was taken at 15 min after the temperature reaches 160 °C.

Fig. 2 shows the TEM images of the CQDs with a mean diameter of 1.403 nm. The present method is found to offer nearly monodisperse nanocrystals without any size selection procedure. In Fig. 2b, the graphitic structure of the CQDs is revealed and the lattice spacing of 0.25 nm matches to the (100) facet of graphite. The dispersity histogram indicates that the standard deviation of the diameter is less than 0.2 nm (Fig. 2c). In Fig. S2, the Raman spectral features, the D band at around 1351 cm⁻¹ and the G band at around 1561 cm⁻¹, are indicated.† The relative intensity of the amorphous D band to the graphitic G band (I_D/I_G) for the CQDs is found to be around 0.5—comparable to that of highly crystalline few layer graphene nanostructures. Thus, we could deduce from the mean diameter that the CQDs are composed of 4 to 7 graphene layers and may be classified as graphene quantum dots or their derivatives.

Fig. 2 (a) A TEM image of nearly monodisperse CQDs. (b) High resolution TEM images of the CQDs reveal lattice spacing of 0.25 nm. (c) A histogram of the diameter distribution shows that the mean diameter and the standard deviation is 1.403 and 0.148 nm, respectively. A low magnification TEM image was used for the population statistics (Fig. S1†).

One common feature of fluorescent carbon nanomaterials is a blue-biased emission spectrum. The emission spectra of the CQDs likewise follows this trend in which the emission peaks are located in the blue region (Fig. 3). Also, the CQDs exhibit excitation wavelength-dependent emission spectra, i. e., the position of the emission peak is dependent on the excitation wavelength (Fig. S3†). The origin of the emission is still unclear; however, earlier works suggest that emissive surface trap sites could be induced by the functional groups (e.g. carbonyl, hydroxyl groups, etc.).^5–15 These functional groups have various energy levels (or exciton binding energies) owing to their distinct chemical properties, which may selectively respond to specific excitation wavelength. In this regard, the surface state of the CQDs is considered crucial to the photoluminescence. In the present method, the surface is grafted by hexadecylamine to form amide groups. The outermost nitrogen atoms would provide more surface trap sites (or raise the surface inhomogeneity) to facilitate the trapping process of photoexcited electrons (or holes). Thus, the quantum yield of the CQDs was recorded as high as 33% (excited at 360 nm) and 42% (420 nm) which is comparable to conventional inorganic nanocrystals.

Fig. 3 Absorption (black) and emission (blue and red for 360 and 420 nm excitation, respectively) spectra of original (dashed lines) and nitric acid-treated (solid lines) CQDs. The concentrations of both suspensions are the same (0.5 mg mL⁻¹).

To further enhance the quantum yield, nitric acid was incorporated in the formation of the CQDs (see ESI†). The nitric acid treatment demonstrated, to our best knowledge, an unprecedented quantum yield of 32% (360 nm) and 53% (420 nm). The absorption spectra with and without the nitric acid treatment exhibit almost the same peak positions, which implies that the size of the CQDs has not significantly changed (Fig. 3).

The preserved size of the CQDs, regardless of the acid treatment, can also be confirmed by the TEM images in Fig. S4.† The duration of the nitric acid treatment was examined over 1 week. Over this period, the nitric acid-treated CQDs preserved the same emission features and their quantum yield slowly decayed to 31% (360 nm) and 47% (420 nm). Since the untreated CQDs also follow the same trend, the degradation over time is likely a result of conventional quenching mechanisms such as photobleaching. This result is promising because the increase in the quantum yield is not a temporary effect.

One plausible explanation for the enhanced quantum yield is oxidation of nonradiative surface trap sites (e.g. dangling bonds, radicals, etc.) which stem from incomplete carbonization. Nitric acid treatment is a well established procedure to introduce oxygen-containing functional groups such as carboxylic acids and alcohols onto the surface of carbonaceous materials.^23–30 Although the mechanism of this reaction is obscure, it is noted that nitric acid plays a central role in the surface oxidation, viz., it is known as the most effective oxidizing agent for carboxylation of nano-sized carbon.^23,24 In this regard, nitric acid incorporated in the formation of the CQDs would oxidize the nonradiative trap sites to carboxyl groups (or other feasible groups). These extra carboxyl groups could be readily passivated by amine capping agents to provide more emissive trap sites and eventually enhance the quantum yield. It is worth noting that the nitric acid treatment changes the shape of the emission spectrum at 360 nm excitation (Fig. 3). The broad emission peak of the untreated CQDs becomes sharpened with the nitric acid treatment. Since the emission feature is directly related to the surface state of nanocrystals, this result may be evidence for the effect of the nitric acid treatment.

To further demonstrate the origin of the enhancement, the effect of some other acids (phosphoric and sulfuric) was examined. Phosphoric acid caused the lowest increase in the quantum yield because it is a weak oxidizing agent compared to the other acids. Although being likewise a strong oxidizing agent and even more acidic than nitric acid, sulfuric acid was found to give an inferior quantum yield to nitric acid. These results imply that there may be a correlation between the acidity and the quantum yield, but it is not simply linear as we expected. This can be attributed to too-high acidity which gives rise to structural damage in the CQDs.^28–30 The emission spectra of all acid-treated CQDs are provided in Fig. S5.†

The nitric acid treatment also caused the emission spectrum to become more “red” relative to the untreated CQDs. The red shift occurs predominantly at 360 nm excitation and the peak position shifts to a longer wavelength by ca. 50 nm (Fig. 5 and S6†). The degree of this shift decreases with the increase of the excitation wavelength and eventually reaches almost zero at 420 nm excitation. The mechanism for this result is not evident, but it is apparent that the change in the surface states induced by the nitric acid treatment is responsible for the red shift. The evidence can be found when the results from the other acid treatments are taken into account. The red shift is a common consequence of all the acid treatments and is most pronounced with nitric acid.

Conclusions

In conclusion, this work demonstrates that highly luminescent nearly monodisperse carbon quantum dots can be prepared by facile emulsion-templated carbonization. The present method is promising because no size selection procedure is required and high quantum yield comparable to inorganic nanocrystals can be achieved. In situ treatments with nitric acid further improve the quantum yield up to 53% (420 nm excitation) which, to our best knowledge, is unprecedented in the past. A plausible explanation for the brightening is that nitric acid could oxidize the surface of the nanocrystals to remove the nonradiative trap sites. This is accompanied with the changes in the emission peak position and shape. To support the result, the effect of the nitric acid treatment on other types of nano-sized carbon materials is compared and some other acids (phosphoric and sulfuric) are also tested. The brightened emission opens prospects for further quantum yield enhancement and the challenging application of carbon quantum dots in solid-state light emitting devices.

Acknowledgements

This research was supported by the Korean Research Foundation (KRF) through the National Research Laboratory Project and POSCO Research Program for Fusion Technology of Materials. The authors thank Hyun-Jin Park and Dr Nam-Suk Lee for their technical assistance in the TEM analysis. W. K. is also grateful to Hyemin Kim for valuable discussion.

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Footnote

† Electronic Supplementary Information (ESI) available: Description of synthesis and characterization and additional figures. See DOI: 10.1039/c2ra22186a

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